I have done something similar to this before, but I really like how it turned out the, in the 12"x12" size, and wanted to try it again.
This time I used an 8"x10" shadow box to create the decor. I used a piece of red and green polka dot card stock to cover the back and filled the shadow box with red and green mini ornaments. I used a digital image from Cricut called "Teresa Collins - Christmas Collage" for my subway art. I sized it at 9"x7," turning off the aspect ratio to make it work. I cut it from white vinyl and put it on the glass of the shadow box. That's all there is to it. This sort of project is really simple, it just takes a while to cut all those words out.